1. Field of the Invention
The present invention relates to a semiconductor device including a protective circuit, and more particularly to a semiconductor device including a protective circuit for preventing breakdown of an insulated gate bipolar transistor which is caused by a stress due to flow of an overcurrent or application of an overvoltage, for example.
2. Description of the Background Art
There has conventionally been a protective circuit which is formed in the same semiconductor device that includes an IGBT (Insulated Gate bipolar Transistor) and functions to protect the IGBT from a stress due to flow of an overcurrent or application of an overvoltage. FIGS. 11 through 13 illustrate such a conventional protective circuit.
FIG. 11 is a circuit diagram of the conventional protective circuit. FIG. 12 is a sectional view of a portion of a semiconductor device in which the conventional protective circuit is formed. FIG. 13 is a plan view of the portion of the semiconductor device illustrated in FIG. 12. The sectional view of FIG. 12 is taken along a line XIIxe2x80x94XII of FIG. 13.
In the conventional protective circuit illustrated in FIG. 11, a terminal P for establishing connection with an external power supply (which will be hereinafter referred to as an xe2x80x9cexternal connection terminalxe2x80x9d) is connected to one end of a resistor R1, the other end of which is connected to a cathode of a first zener diode D1 and a semiconductor element such as an IGBT not shown. An anode of the first zener diode D1 is connected to any kind of a constant potential node such as a ground.
Next, a structure of the semiconductor device in which the conventional protective circuit illustrated in FIG. 11 is formed will be described with reference to the sectional view of the semiconductor device in FIG. 12.
An n+-type semiconductor layer 2 is formed on a p-type semiconductor substrate 1 by using epitaxial growth. On the n+-type semiconductor layer 2, an nxe2x88x92-type semiconductor layer 3 is formed by using epitaxial growth. An oxide film 4 is formed on the nxe2x88x92-type semiconductor layer 3, and a polysilicon region is provided in a portion of the oxide film 4. A p-type diffusion layer 5 and an n+-type diffusion layer 6 are formed in respective predetermined portions of the polysilicon region by diffusing impurities into the corresponding portions.
Further, an insulating film 7 is formed so as to cover respective top faces of the oxide film 4, the p-type diffusion layer 5 and the n+-type diffusion layer 6. Contact holes extending from a surface of the insulating film 7 and respectively reaching the diffusion layers 5 and 6 are formed in respective predetermined portions of the insulating film 7. Each of interconnects 8 and the external connection terminal P is formed by filling each of the contact holes with a conductor such as a metal in a predetermined pattern.
Moreover, an electrode 10 used for an IGBT or the like is formed on a back face of the p-type semiconductor substrate 1.
In the semiconductor device with the foregoing structure, a junction between the p-type diffusion layer 5 and the n+-type diffusion layer 6 forms the first zener diode D1, while the n+-type diffusion layer 6 which connects one of the interconnects 8 and the external connection terminal P forms the resistor R1.
The one of the interconnects 8 which is connected to the n+-type diffusion layer 6 is to be connected to the IGBT, while another one of the interconnects 8 which is connected to the p-type diffusion layer 5 is to be connected to a constant potential node.
FIG. 13 is a plan view of the foregoing configuration, in which the insulating film 7 covering the diffusion layers 5 and 6 are omitted for purposes of clarifying the formation of the first zener diode D1 and the resistor R1.
In the protective circuit with the above-mentioned configuration, the first zener diode D1 is formed in order to protect the IGBT from a stress due to application of an overvoltage which is induced by a surge such as static electricity externally supplied.
More specifically, upon application of an overvoltage which is induced by a surge such as static electricity via the external connection terminal P, a zener breakdown takes place in the first zener diode D1, to absorb the overvoltage induced by a surge as applied. This prevents a voltage equal to or higher than a breakdown voltage from being applied to the IGBT. Accordingly, it is possible to prevent the IGBT from being broken down under a stress due to application of an overvoltage which is induced by a surge.
On the other hand, the resistor R1 is formed in order to protect the first zener diode D1 from a stress due to flow of an overcurrent from an external power supply.
More specifically, the resistor R1 is formed in order to provide for occurrence of an event where an external power supply Vd for driving the IGBT is improperly connected in a direction contrary to a normal direction as illustrated in FIG. 14. In such an event, even if a direct current I continues to flow from the external power supply Vd for a predetermined time, a value (flow rate) of the direct current I is limited to a degree where the first zener diode D1 is not broken, by the resistor R1.
As described above, according to the conventional protective circuit, the first zener diode D1 is formed in order to protect a semiconductor element such as an IGBT from a stress due to application of an overvoltage which is induced by a surge such as static electricity, and the resistor R1 is formed in order to protect the first zener diode D1 from a stress due to flow of an overcurrent which may possibly be caused by improper connection of the external power supply Vd.
However, if an overvoltage is applied to the conventional protective circuit including the resistor R1 and the first zener diode D1 as illustrated in FIG. 11 because of occurrence of a surge, a voltage difference which is equal to a difference between the applied overvoltage and a breakdown voltage is provided between opposite ends of the resistor R1.
As a result, an electric power is generated due to the voltage difference between the opposite ends of the resistor R1 so that the resistor R1 is excessively heated. Thus, if an overvoltage equal to or higher than a predetermined voltage is applied to the protective circuit because of occurrence of a surge, the resistor R1 is likely to be burnt and be disconnected from the protective circuit. Thus, because of the formation of the resistor R1 within the protective circuit, an entire voltage tolerance of the protective circuit to a stress due to application of an overvoltage induced by a surge is degraded.
In order to improve the voltage tolerance of the protective circuit including the resistor R1 to a stress due to application of an overvoltage induced by a surge, minimization of a resistance of the resistor R1 may be effective on one hand. However, on the other hand, minimization of a resistance of the resistor R1 would reduce a current (flow rate) tolerance of the protective circuit for protecting the first zener diode D1 from a stress due to flow of an overcurrent which is induced by improper connection of an external power supply. In view of this, there are limits to how much the voltage tolerance and the current tolerance can be improved, respectively.
It is an object of the present invention to provide a semiconductor device including a protective circuit capable of improving a voltage tolerance of the protective circuit while maintaining a predetermined current tolerance of the protective circuit.
According to the present invention, a semiconductor device with a semiconductor element formed on a semiconductor substrate and a protective circuit for the semiconductor element includes a resistor, a first zener diode and a plurality of second zener diodes. The resistor has one end connected to a terminal for external connection and the other end connected to the semiconductor element. The first zener diode is connected between the other end of the resistor and a constant potential node. The plurality of second zener diodes are connected in series between the one end of the resistor and the constant potential node. The number of the plurality of second zener diodes is larger than that of the first zener diode.
The number of the second zener diodes to be connected is controlled so as to allow a sum of forward rising voltages of the second zener diodes to be higher than a voltage of an external power supply connected with the terminal for external connection. This ensures that a current flowing from the external power supply flows into the resistor via the first zener diode thereby to advantageously reduce the current, not allowing the current to flow into the second zener diodes even if the external power supply is improperly connected in a direction contrary to a normal direction.
Accordingly, it is possible to prevent the first and second zener diodes from being broken due to flow of a current from the external power supply.
Also, even if an overvoltage is applied via the terminal for external connection because of occurrence of a surge such as static electricity, the opposite ends of the resistor are kept at respective constant potentials and an excessive voltage difference is not provided between the opposite ends of the resistor. Accordingly, the resistor is saved from burning due to application of an overvoltage induced by a surge.
Thus, it is possible to improve a voltage tolerance of the protective circuit while maintaining a current tolerance thereof.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.